It is known that with closures having a metal shell and a depending skirt that the shell should include a liner made of a synthetic resin which is press-molded to the underside of the shell in order that the closure may obtain a tight seal with a container opening. A variety of liner shapes press-molded to the underside of a metal shell have been proposed. We have found that one desirable shape of a liner includes at least one circular projecting rim adapted to tightly seal with the surface of a container opening such as the liner disclosed in Japanese Patent Early Disclosure No. 53-65184 published in the 1978 Japanese Patent Gazette.
It is known that the cost of a closure construction such as described in the aforementioned Japanese Early Disclosure and pending application can be kept low if the liner can be manufactured as a single unit and if the amount of synthetic resin material used in forming the liner can be minimized. Usually the outside circumferential surface of the projecting rim of the liner is positioned to establish an interior space along a radial axis extending between the liner and the skirt of the shell (in situations where the liner itself has two or more projecting rims, the outer circumferential surface is established on the outermost projecting rim), and this space is contained between the outside circumferential surface of the liner and the inside surface of the shell skirt. If the space is large, then the required quantity of synthetic resin material will be reduced, but as the thickness of the projecting rim along a radial axis decreases and the space itself increases, then the strength of the projecting liner rim will also decrease. This space between the outside circumferential surface of the projecting liner rim and the inside of the metal shell skirt is thus limited in range by the size or degree of space required.
Under these circumstances, prior liners have been press-molded to a shell's underside surface in such a shape as shown in FIG. 1. Where the closure shown comprises a shell 2 formed from a sheet of some suitable metal such as aluminum alloy, tin plate, chromed steel, etc. and a liner 4 formed from an appropriate synthetic resin such as polyethylene, polyolefin, etc. The metal shell 2 has a circular upper surface 6 and a skirt 8 which depends from the circumference of the circular upper surface 6 (only a portion of this skirt is shown in Drawing 1). Based on typical methods of manufacturing for this type of closure, a liner 4 can be press-molded to the underside of the circular upper surface of the shell, and this liner has at least one downwardly projecting rim, or as shown in FIG. 1, two concentric projecting rims including an outer circular projecting rim 10 and an inner circular projecting rim 12. Such methods of manufacturing are described in Japanese Patent No. 40-13156 in Patent Gazette 1965, No. 41-5588 in Patent Gazette 1966, No. 48 -5706 in Patent Gazette 1973 and No. 48-19886 in Patent Gazette 1974, and also in U.S. Pat. Nos. 3,135,019; 3,212,131 and 3,278,985. Circular projecting rims 10 and 12 are situated in the closure so that they are adapted to tightly contact a container opening (not shown in Drawing 1) by closing against the outer circumferential surface, top surface or inner circumferential surface of the opening thus causing the container to be tightly sealed. The outside circumferential surface of the outer projecting rim 10 of the liner 4 is positioned so as to create a space extending inwardly along a radial axis extending an appropriate distance d.sub.1 from the inside circumferential surface at the beginning point of the skirt 8 of the metal shell 2 with the result that the space extends between the outside circumferential surface of the outer projecting rim 10 and the inside circumferential surface of the metal shell skirt 8.
We have discovered several problems which exist in closures constructed as shown in FIG. 1. With the liner configuration of FIG. 1, provision must be made to adhere the liner to the underside of the circular surface 6 of the metal shell when it is press-molded as well as afterwards. We have found that based on the time which elapses from the moment when the liner is press-molded, there is a tendency for the outer edge of the liner to gradually peel off from the underside of the circular surface of the metal shell. The reasons why such frequent peeling occurs have not been precisely determined, but it may originate from the pressure under which the liner material flows when the liner is press-formed. When the liner is press-formed in the shape shown in FIG. 1, a residual stress is imparted which remains in the liner to act along a radial axis as shown by directional arrow 14. We believe that the outer edge of the liner tends to peel away from the circular surface of the metal shell because of such residual stress acting in concentration on the outer projecting rim 10 and the inner projecting rim 12.
It is known that when a closure which has a liner with the configuration shown in FIG. 1 is installed on the mouth of a container to close it tightly, due to a variety of reasons, a relatively large amount of stress is added to the liner projecting rims 10 and 12 thus resulting in a fair amount of cracking of the rims. However, if the liner properly contacts the circular surface, there will be few problems when this type of cracking develops because the inside of the container extends to the region where the cracking and peeling off occurs. However if peeling takes place as described above and the region where the cracking and peeling occurs inside the container also extends to the outside of the container, a tight seal on the mouth of the container will be destroyed.
It is therefore an object of our invention to provide for a closure construction having a liner configuration which will prevent peeling of the outer edge of the liner from a circular surface of a metal shell over a period of time.